As a matter of course, mechanical components support civilization and form the foundation of industries, and it is not an exaggeration to say that the fundamental working principles of a gear mechanism as one type of mechanical component had all been mostly invented in Greek or Roman times and led the way to the present. Therefore, during these last one hundred years, the only notable inventions are harmonic gears and mechanical paradox gears. In gear mechanisms having such a long history, it is not too much to say that high efficiency and high reduction gear ratios are eternal dreams. Gear mechanisms which are structurally simple, realize high reduction gear ratios, great torque transmission, and high efficiency have been pursued for many years and many researchers have been engaged in the realization of these.
Recently, according to the present inventions of harmonic gears, the reduction gear ratio and efficiency of the mechanisms have been improved to some degree, and at present, these are dominant in many mechanical fields. However, the harmonic gears have fewer teeth meshes, and they are not suitable for transmitting a great torque. Therefore, there is great expectation for mechanical paradox gears as one type of conventional planetary gear mechanism which includes more teeth meshes, does not require special gears, and can be easily downsized.
The mechanical paradox gear mechanism (Furgusson' s Mechanical Paradox Gear) includes two gears having different numbers of teeth attached to one shaft mesh with another common gear, and from a long time ago, it has been used for variable capacitor dial mechanisms for radios and propeller variable pitch mechanisms, etc. A planetary gear using the mechanical paradox gear mechanism is a mechanical paradox planetary gear device, and M. Morozumi has studied this. The mechanical paradox planetary gear is superior to the harmonic gear in the point that the mechanical paradox planetary gear realizes an extremely great reduction gear ratio by a simple structure, includes more teeth to mesh with each other, and can transmit a great torque.
Recently, a mechanical paradox planetary gear has been made practicable as an extremely small gear to be used in a printer or the like, and mechanical paradox planetary gears have attracted wider attention, however, in principle, it has been a mechanism in which a pair of gears mesh with each other, so that it has been pointed out that the mechanism is still insufficient in efficiency due to its low-strength teeth and small transmittable torque, and, it must be said that the degree of completion as a gear mechanism is low.    Patent Document 1: Japanese Published Unexamined Patent Application No. 2000-274495    Patent Document 2: Japanese Published Unexamined Patent Application No. 2004-19900    Patent Document 3: Japanese Published Unexamined Patent Application No. 2003-194158    Patent Document 4: Japanese Published Unexamined Patent Application No. 2001-317598    Non-Patent Document 1: Muneharu Morozumi, “Design Calculation of Planetary Gear and Differential Gear,” KABUSHIKIKAISHA SANKEISHUPPANSHA, June 1984    Non-Patent Document 2: Takashi Nakata, “Profile Shifted Gear,” The Japan Society of Mechanical Engineers, February 1994